In a large saucepan heat oil until hot. Add onions; saute until transparent, about 5 minutes. Add water, thyme, paprika, black pepper and vinegar; bring to a boil. Add celery leaves, celery and potatoes. Simmer, covered, until vegetables are very soft, about 15 minutes. Remove from heat. Remove celery leaves. Puree half of the vegetables and liquid in the container of an electric blender or a food processor fitted with a steel wing blade until smooth. Return to saucepan. Heat until hot.